1. Field of the Invention
The present invention relates to remotely controlled dimmable electronic ballasts for powering gas discharge lamps and, in particular, to a ballast system which utilizes a power line interruption coding system and means for controlling the lamp filament voltage such that as the lamps are dimmed, the filament voltage is increased
2. Description of the Prior Art
The fluorescent lamp is designed to be a replacement for the incandescent lamp. The fluorescent lamp offers very large energy savings as compared to incandescent lamps. For example, a 28 watt fluorescent lamp offers the same light as a 100 watt incandescent lamp. This tremendous energy savings has been ignored in some applications because of the nonavailability of appropriate ballasts and control systems. For example, special lighting in restaurants, hallway lights, and other areas wherein the light level needs to be controlled for either energy savings or special effects has in the past used incandescent lamps with energy-wasteful dimming systems to obtain the desired effect. Incandescent dimming systems utilize either variable transformers, triacs or electronic means. The electronic means are the most cost-effective but have serious drawbacks in the form of a very low power factor, low efficiency and increased harmonic generation. The energy that may be saved by reducing the kilowatts delivered to the lamp load is utilized because of a low power factor and high harmonic generation. Since low power factor and high harmonics are harmful to the power system, power companies continue to search for ways to give their customers the lighting aesthetics they desire while still saving energy.
A dimming system would require adding extra wires in the wall to connect the controls and switches to the dimming ballasts. This is generally unacceptable, since it is very expensive and thus the prior art sought to communicate to the ballasts in a different manner. Prior art systems involved using carrier current type communications over the power line. A well-designed carrier current type system generally will work reliably in even difficult conditions. However, the greatest drawback with this type of communication system is that it is a broadcast system, i.e. signals are transmitted in all directions along the wires and therefore are required to carry complex coding information. In addition the ballasts themselves are required appropriate decoding or addressing circuitry. The overall modification is costly and requires a much larger wall switch box to accommodate the additional equipment. In addition to the communication problem noted, the prior art has sought to provide cost efficient techniques for controlling dimming of the fluorescent lamps. One of the approaches uses a frequency change method (shift) to both control the lamp current (power) and to maintain the lamp filament voltage substantially constant as the lamps were dimmed in order to maintain lamp life. Frequency dimming circuitry, however, adds to the overall cost and complexity of the dimming ballast system. A less costly prior art technique utilizes a variable voltage power source to control lamp dimming. However, filament voltage could not be controlled by a simple circuit to preserve lamp life, thus making the technique commercially unfeasible.
Typical of the prior art ballast control systems are those disclosed in U.S. Pat. No. 4,717,863 to Zeiler wherein a frequency modulated circuit is utilized to provide a variable voltage to dim the fluorescent lamp, an optical feedback system being utilized to regulate the frequency of the output signal; U.S. Pat. No. 4,523,128 to Stamm et.al. which discloses a system for the remote control of a solid state ballast which interfaces with a power line carrier system to provide external addressing control signals, the control system including a signal receiver for receiving and recognizing remotely transmitted control signals addressed to the ballast; U.S. Pat. No. 4,889,999 to Rowen which discloses a control system wherein information is transmitted to individual dimmer controls by extra wires, the dimmer controls using a triac to control the voltage to the ballast to dim the light output; U.S. Pat. No. 4,388,567 to Yamazaki et.al. which discloses a system for remotely controlling a dimming apparatus which uses single phase control to vary the voltage and therefore control light output; U.S. Pat. No. 4,388,563 to Hyltin which discloses a solid state fluorescent lamp ballast circuit in which line voltage is chopped to provide a high frequency input to a fluorescent lamp, the duty cycle of the chopping switches being modulated to permit dimming of a remotely located lamp; and U.S. Pat. No. 4,866,350 to Counts which discloses a system wherein power is provided to a fluorescent lamp through a single integrated circuit chip, control logic within the chip operating power switches therein at a frequency which is optimum for the fluorescent lamp.
Although the aforementioned prior art systems provide various features which improve upon the ballast used in fluorescent lighting systems, they all suffer in one way or the other from the disadvantages noted hereinabove, i.e. requiring a carrier current type encoding system and/or lamp dimming techniques which are costly, complex and not commercially viable.